The Influence of Arc Roots on Current Interruption

Summary

Until now it has not been possible to determine a simple relationship between the interruption capability of a circuit breaker and the arc-root behavior. Observation of the roots of a high-current arc on WCu-electrodes in a model circuit breaker reveals a strong vaporization of electrode material. Since contamination of the quenching medium by several percent of metal vapor increases its electrical conductivity at low temperatures (2000–6000 K) by at least an order of magnitude, this metal vapor should decrease the quenching ability. It is also observed that frequent separation of the arc and the jet of electrode vapor occurs, accompanied by removal of the arc roots from the ignition point by the vapor. Accordingly mixing of the arc plasma and the vapor does not take place. To clarify this behavior, spectroscopic measurements were made of the plasma- and vapor temperature in front of the graphite cathode of a high-current dc-arc. The measurements show that in the case of strong vaporization, the electrode vapor in the vicinity of the electrode is much colder than the arc plasma. By using an appropriate electrode material, it is possible to avoid the contamination of the quenching medium and uncontrolled movement of the arc roots.